stock unwinding from GotW #47

It is per Sutter's GotW #47 article.  I don't understand why if a U
object is destroyed due to stack unwinding during to exception
propagation, T::~T will fail to use the "code that could throw" path
even though it safely could.

Code from the article:

   //  The wrong solution
    //
    T::~T() {
      if( !std::uncaught_exception() ) {
        // ... code that could throw ...
      } else {
        // ... code that won't throw ...
      }
    }


   //  Why the wrong solution is wrong
    //
    U::~U() {
      try {
        T t;
        // do work
      } catch( ... ) {
        // clean up
      }
    }